Von Mises Values and Code Allowables

[wlc1]

I do not have a copy of Section VIII, Div 2 and have been given a support lug review done using a FEA analysis. The stress values presented are given as Von Mises stresses. How does one compare these stresses to the allowables in Section II Part D? Does the ASME code use Von Mises or Tresca?. Any help or good references for this?

 

[jte]

wlc1-

Von Mises and 2*Tresca will usually be within around 10% of each other, so if the FEA results have a comfortable margin in Von Mises, then you'll be ok. Shouldn't be a big deal for your FE analyst to kick out some stresses in stress intensity, though. I'd ask them to provide you the analysis using stress results which can be directly compared to the Code.

References: VIII Div 2 4-112(a) ...the stress intensity, is defined as twice the maximum shear stress...

When the new Div 2 is issued (summer '06 - anyone have an update?) stress intensity will be dropped in favor of Von Mises.

jt

 

[TGS4]

I agree with jte's assessment comparing von Mises vs. Tresca.

However, when I review a Div. 2 FEA report that has von Mises stresses in it, alarm bells start to go off. This is a very good indication that your FE analyst knows NOTHING about Div. 2 analyses. I also suspect that they did not bother to provide you with membrane and membrane-plus-bending stress intensities either (or if they did, they probably did it wrong...). I am also quite sure that they didn't check to ensure that the extent to which the local membrane stress intensity exceeds 1.1*Sm was less than 1*sqrt(R*t) in the meridional direction. I've seen this so many times that I've become quite the cynic in this respect...

As far as references go, I would suggest that you take a read through Appendix 4 yourself. If you have any specific questions about it, please come back here and we will gladly answer your questions.

To answer jte's question about Div. 2 rewrite - the plan is to ballot (or at least start to ballot) in January. I wouldn't hold my breath about it hitting the street in '06 though...